Saturated magnetic flux density of a core material used in armature cores or field cores generally occurs at 19,000 to 21,000 gauss or so.
Miniaturized permanent magnets having mighty electromotive forces have been developed recently; however, the magnetic flux density of the surface of these permanent magnets is in the range of 2,500 to 3,000 gauss or so in most magnets formed of ferrites. Even with magnets formed of rare earth elements to ensure the highest magnetic flux density, the flux density on their surfaces runs only at 6,000 to 8,000 gauss or so. Therefore, the core material may allow the magnetic flux of a magnet using rare earth elements which are about three times as large as the core in area, to pass therein.
Consequently, for miniaturization of an electrical rotary machine having permanent magnets, it is essential that an arrangement of the permanent magnets will be taken into consideration so as to dispose the permanent magnets with the largest possible pole-face area in the smallest possible space.
In view of the above requirements, the applicant proposed an arrangement of permanent magnets for pole cores as disclosed in Japanese Patent Laid-Open No. 77359/1980.
The structure is such that a permanent magnet of circular arc in section rectangular to an axis of rotation is disposed with its one end facing near a rotating shaft and the other end facing near the outer periphery of a pole core, therefore an electromotive force waveform reaching an armature core includes many an odd harmonic, which does not ensure a good characteristic in electrical rotating machinery.